Imagers, such as for example CCD, CMOS and others, are widely used in imaging applications, for example, in digital still and video cameras.
It is well known that, for a given optical lens, IR filter, housing, etc, comprising a digital still or video camera, the pixels of the pixel array will generally have varying levels of output signal even if the imaged scene is uniform. A camera may have non-uniform pixel response, depending on a pixel's spatial location within the pixel array. One source of such variations is lens shading. Lens shading causes pixels in a pixel array located farther away from the center of the pixel array to have a lower value when compared to pixels located closer to the center of the pixel array, when exposed to the same level of light stimulus. These variations can be compensated for by adjusting, for example, the gain of the pixels based on spatial location in a pixel array. For lens shading correction, for example, it may happen that the further away a pixel is from the center of the pixel array, the more gain is needed to be applied to the pixel value. Different color channels of an imager may also be affected differently by varying lens shading patterns.
In addition, sometimes an optical lens is not centered with respect to the optical center of the imager; the effect is that lens shading may not be centered at the center of the imager pixel array. Other types of changes in optical state and variations in lens optics may further contribute to a non-uniform pixel response across the pixel array. For example, variations in iris opening or focus position may affect pixel response.
Variations in the shape and orientation of photo sensors used in the pixels may also contribute to a non-uniform spatial response across the pixel array. Further, spatial non-uniformity may be caused by optical crosstalk or other interactions among pixels in a pixel array.
Sometimes response variations caused by the spatial position of a pixel in a pixel array can be measured, and the pixel response value can be corrected with an adjustment. Lens shading, for example, can be corrected using a set of positional gain adjustment values which adjust pixel gain in post-image capture processing of the pixel values. Typically, adjustments across the pixel array can be provided as pixel correction values, one or more corresponding to each of the pixels. The pixel correction values are applied to the pixel values during post-image capture processing to correct for pixel value variations due to the spatial location of the pixels in the pixel array.
Pixel correction values may be determined by correction functions produced from stored function parameters. The pixel correction values dependent upon the pixel's location in the pixel array are used to determine a corrected pixel value, P(x, y). One method of determining a corrected pixel value, P(x, y), is disclosed in co-pending U.S. patent application Ser. No. 10/915,454, entitled CORRECTION OF NON-UNIFORM SENSITIVITY IN AN IMAGE ARRAY, filed on Aug. 11, 2004, (“the '454 application”), which is incorporated herein in its entirety by reference and is briefly described below. Disclosed embodiments provide an alternative method for correcting pixel values using a reduced amount of memory, simplifying processing, allowing the pixel correction values to be more easily calculated in real time, and/or providing a more general correction surface to be generated than in the '454 application.